css文件：
* {
    padding: 0;
    margin: 0;
    box-sizing: border-box;
    font-family: 'Poppins',sans-serif;
}

body {
    display: flex;
    justify-content: center;
    align-items: center;
    min-height: 100vh;
    background-color: #23242a;
}


.box {
    width: 380px;
    height: 420px;
    position: relative;
    background-color: #1c1c1c;
    border-radius: 8px;
    overflow: hidden;
}


.box::before {
    content: '';
    position: absolute;
    width: 380px;
    height: 420px;
    left: -50%;
    top:-50%;
    background: linear-gradient(0deg,transparent,transparent,#ff2770,#ff2770,#ff2770);
    z-index: 1;
    animation: animate 6s linear infinite;
    transform-origin: bottom right;
}

.box::after {
    content: '';
    position: absolute;
    width: 380px;
    height: 420px;
    left: -50%;
    top:-50%;
    background: linear-gradient(0deg,transparent,transparent,#ff2770,#ff2770,#ff2770);
    z-index: 1;
    animation: animate 6s linear infinite;
    transform-origin: bottom right;
    animation-delay: -3s;
}

@keyframes animate {
    from{
        transform: rotate(0deg);
    }

    to{
        transform: rotate(360deg);
    }
}

.divline {
    position: absolute;
    top: 0;
    inset: 0;

}

.divline::after {
    content: '';
    position: absolute;
    width: 380px;
    height: 420px;
    left: -50%;
    top:-50%;
    background: linear-gradient(0deg,transparent,transparent,#27ff85,#27ff85,#27ff85);
    z-index: 1;
    animation: animate 6s linear infinite;
    transform-origin: bottom right;
    animation-delay: -1.5s;
}

.divline::before {
    content: '';
    position: absolute;
    width: 380px;
    height: 420px;
    left: -50%;
    top:-50%;
    background: linear-gradient(0deg,transparent,transparent,#27ff85,#27ff85, #27ff85);
    z-index: 1;
    animation: animate 6s linear infinite;
    transform-origin: bottom right;
    animation-delay: -4.5s;

}


.box form {
    position: absolute;
    inset: 2px;
    border-radius: 8px;
    background-color: #222;
    padding: 50px 40px;
    z-index: 2;
    display: flex;
    flex-direction: column;
}



.box form h2{
    color: white;
    font-weight: 500;           /*字体粗细*/
    letter-spacing: .1rem;      /*控制字体间隔为根元素的十分之一*/
    text-align: center;
}


.box form .input-box {
    position: relative;
    width: 300px;
    margin-top: 35px;
}

.input-box input {
    position: relative;
    border: none;
    outline: none;
    background: transparent;
    padding: 20px 10px 10px;
    width: 100%;
    box-shadow: none;
    color: #23242a;
    z-index: 10;
    font-size: 1rem;
    letter-spacing: 0.05rem;
    transition: .5s;
}

.input-box span {
    color: #8f8f8f;
    font-size: 1rem;
    position: absolute;
    left:0;
    padding: 20px 0 10px;
    transition: .5s;
    letter-spacing: 0.05rem;
    pointer-events: none;
}

.input-box input:valid~span,
.input-box input:focus~span {
    font-size:0.75rem;
    transform: translateY(-34px);
    color: white;

}

.input-box i{
    position: absolute;
    left:0;
    bottom: 0;
    height: 2px;
    background-color: white;
    width: 100%;
    border-radius: 4px;
    pointer-events: none;
    overflow: hidden;
    transition: .5s;
}

.input-box input:valid~i,
.input-box input:focus~i {
    height: 44px;

}

.links {
    display: flex;
    flex-direction: row;
    justify-content: space-between;
    margin: 10px 0;
}

.links a {
    text-decoration: none;
    font-size: 0.65rem;
    color: #8f8f8f;
    letter-spacing: 0.01rem;
    transition: 0.3s;
}

.links a:hover,
.links a:nth-child(2) {
    color: white;
}

form input[type="submit"] {
    width: 100px;
    border-radius: 4px;
    outline: none;
    border: none;
    background-color: white;
    padding: 9px 25px;
    cursor: pointer;
    font-size: 0.9rem;
    margin-top: 10px;
    transition: .1s;

}



/*----------------------------*/
/*
.input-box {
    position: relative; !* 确保子元素可以使用绝对定位 *!
    padding-bottom: 10px; !* 为下划线留出空间 *!

}

.input-box i {
    position: absolute;
    left: 0;
    bottom: 0;
    height: 2px;
    width: 0; !* 初始宽度为0 *!
    background-color: white;
    transition: width 0.3s ; !* 平滑过渡宽度变化 *!
}

.input-box i:before {
    content: ''; !* 必须设置content属性 *!
    position: absolute;
    left: 0;
    bottom: 0;
    height: 2px;
    width: 100%; !* 初始宽度为100% *!
    background-color: #ff6f95; !* 设置颜色 *!
    transition: width 0.3s ease, background-color 0.3s ease; !* 平滑过渡宽度和背景颜色变化 *!
}

!* 当.input-box获得焦点时改变<i>和<i:before>的样式 *!
.input-box:focus-within i,
.input-box:hover i {
    width: 100%; !* <i>标签的宽度变为100% *!
}

.input-box:focus-within i:before,
.input-box:hover i:before {
    width: 0; !* <i:before>的宽度变为0 *!
    background-color: white; !* 改变背景颜色 *!
}

.input-box input {
    color: white;
}*/
